Bounties are the way to earn xp, not ordinary mobs. Bounties reset every "day" (22 hours to be exact).

Whilst the xp you get for killing bounties again each day is less than the first time bonus, going back and killing as many bounties as possible each day is the best way to level quickly. (Certainly the ones close to your level - killing bounties 20 levels below you will not help xp much.)

Once you hit level 40 ish, there are zones which require you to collect medallions from bounties to obtain keys to locked zones, so you will need to run the Traven zones two or three times anyway.

Killing bounties daily also earns you CoG coins (exchangeable for supplies) and bounty power which helps with gathering, so there is a double benefit to doing regular bounty runs. All bounties give CoG coins, but only the ones close to your level give bounty power.